Previously to this end, for example, when the aircraft for checking has been on the ground, samples of the hydraulic fluid have been taken and these samples have been sent to chemical laboratories in order to be able to check the quality of the hydraulic fluid. On the one hand, such analysis takes a long time and can generate high costs. On the other hand, samples of the hydraulic fluid cannot be taken constantly, and therefore continuous monitoring of the hydraulic fluid cannot be ensured by such a method.
It is therefore desirable to use monitoring methods which can monitor the hydraulic fluid continuously in respect of its quality, without the aircraft having to be on the ground.
For example, sensors are used which record the electrical or dielectric properties of the hydraulic fluid. On the other hand, however, optical sensors are preferable since the recorded data correlate directly with the chemical properties such as the water content and the acid number. The Applicant has previously experimented in this field with sensors which expose the hydraulic fluid to light from the mid-infrared range (3000 nm to 5000 nm), so as to acquire the absorption spectra of the exposed hydraulic fluid.
These absorption spectra can be used to determine the chemical composition of the hydraulic fluid.